The present invention relates to a drive unit for a rotary decoration used, for example, in a table clock.
A popular, highly decorative and fashionable clock is a table clock having a rotary decoration. A rotary decoration constituting the primary decorative component of such a table clock is usually mounted rotatably or turnably on a vertical member or shaft, so as to undergo a swing movement about the vertical shaft in a horizontal direction. A hair spring or a motor is used for causing a reciprocative swing movement of the rotary decoration.
One example of a prior art driving means for a rotary decoration is a hair spring, which is constituted in such a manner that a rotary decoration is supplied a reciprocative swing movement by the application of the balance-wheel motor principle used in a transistor clock, while its rotational shaft is driven Via the hair spring to cause a moderate sine curve, thus imparting an elegant motion to the rotary decoration. Japanese Utility Model Publication No. 21821 (1985) discloses such a clock.
However, the hair spring type rotary decoration is disadvantageous in that it involves a complicated design and manufacturing process and, therefore, is not suited for mass production. Furthermore, the cost of parts is high. This type of rotary decoration is also troublesome to install, since careful attention is required in installing the hair spring.
As another example of a driving means for a rotary decoration, the present inventors have developed a stepping motor for a rotary decoration which is disclosed in Japanese Patent Application No. 43718 (1991). This stepping motor is capable of rotating or turning in both the forward (normal) and reverse directions, and the drive timing of the motor is controlled in such a manner that the rotary decoration exhibits a reciprocative swing movement on the same sine curve as in the case of the hair spring type.
The drive unit using the reciprocating stepping motor described in the second example uses a damper, such as a coil spring, in the transmission gear train so that a smooth rotational motion can be obtained in the decoration regardless of the characteristics of the stepping motor. However, this arrangement has the following three problems:
(1) In order to speed up the rotation period without varying the maximum swing angle .theta.1, it is normally required to reduce the interval d of an output pulse used to drive the stepping motor. In other words, it is required that the pulse interval d be shortened so that numerous drive pulses can be generated within a short period of time. In view of the physical characteristics of the stepping motor, the minimum pulse interval d.sub.min is restricted to a predetermined value. Therefore, the maximum pulse interval d.sub.max must be made smaller.
If the maximum pulse interval d.sub.max is reduced to a smaller value while d.sub.min is limited to its predetermined minimum value, the difference between the maximum pulse interval d.sub.max and the minimum pulse interval d.sub.min becomes small. Hence, the difference between the angular velocity .omega..sub.max of the rotary decoration during low-speed rotation and the angular velocity .omega..sub.min of the rotary decoration during high-speed rotation likewise becomes small. Thus, there is no crispness in the rotation of the rotary decoration.
Furthermore, if the maximum pulse interval d.sub.max is reduced to a smaller value, the angular velocity .omega. cannot be sufficiently reduced when the rotary decoration changes its rotational direction. Namely, the rotary decoration starts to move fast from the beginning. As a result, the motion of the rotary decoration looks visually awkward. Therefore, there is a problem in the prior art wherein the desired elegant swing movement of the rotary decoration cannot be achieved.
Moreover, if numerous drive pulses are generated within a limited short period of time, electric power consumption increases, and the life of the battery is thus shortened.
(2) When the rotary decoration changes its rotational direction, a coil spring is wound up. At the beginning of this winding operation, the rotary decoration is stopped for a period, due to the fact that the coil spring is unwound, and then wound up again. Accordingly, it is not possible to impart the desired elegant swing movement to the rotary decoration.
(3) Even if the application of the drive pulses for the stepping motor is suddenly terminated, the wound-up coil spring is gradually loosened. Therefore, the spring force stored in the coil spring is gradually transferred to the rotary decoration. Thus, the rotary decoration continues to rotate for a time due to its inertia.
Accordingly, even if a drive pulse is suddenly outputted to cause the stepping motor to rotate in the reverse direction in an attempt to reverse the rotational direction of the rotary decoration, the rotary decoration maintains its current rotational direction for a period of time by virtue of its inertia. As a result, the rotational driving force of the stepping motor and the inertia derived from the coil spring acting in the reverse direction interfere with each other. Thus, irregularities arise in the motion of the rotary decoration.